Removable plugging devices have long been available in the plumbing and steamfitting arts for temporarily sealing pipelines to allow them to be drained for repair or pressure tested for leaks. In recent years a particular need for reliable pipeline plugs has arisen in nuclear power plants where pressure tests must periodically be carried out to assure that no leakage occurs in steamlines leading from the boiling water reactor. Such tests are typically conducted as completion of the refueling down time approaches and before reinstalling the reactor vessel head in order to reduce radiation exposure and outage, just prior to start-up of the plant. In the course of the tests, a removable plug is inserted in the main steam line to seal it off against pressure from either direction, which is to say from the reactor vessel to the pipeline or from the pipeline to the vessel.
In some prior art removable plugs, the locking action involves axially squeezing together opposed elements so as to increase the diameter of some expandable component between them. The component increasing in diameter engages the interior of a pipeline to hold the plug in place or create the seal or both. A rubber ring is axially compressed by wedged surfaces in accordance with this principle in U.S. Pat. No. 3,693,408. The axial forces which squeeze and expand a plug into the pipe in such prior art teachings may be generated by torquing a nut or by pressurizing a hydraulic cylinder or otherwise, and the component therebetween which expands in diameter may comprise metal shoes or be of resilient material. Inflatable annular seals are sometimes used alone as in U.S. Pat. No. 3,919,880, or in conjunction with expandable shoes as in U.S. Pat. No. 3,593,749.
In any case the plugs of the prior art have particular disadvantages which the present invention seeks to overcome. It is characteristic of all of the prior art squeeze-type plugs that given irregularities in pipeline dimensions it is practically impossible to determine accurately the radial locking force of the plug in the pipe and thus its true pressure retaining capability. Very few pipes are precisely round and when metal locking shoes are expanded radially in prior art plugs they move outwardly together to engage different areas of the pipeline interior with varying force, thereby generating concentrated loads which may severely stress the pipe wall. On the other hand creeping of the plug along the pipe sometimes occurs with a resulting pressure loss in the test section when rubber rings alone are relied upon to fix the plug in place and create the seal.